1. Field of the Invention
The present invention relates to a process of forming a coherent refractory mass on a surface by projecting against this surface, simultaneously with oxygen, a mixture of refractory particles and combustible particles which react in an exothermic manner with the projected oxygen and release sufficient heat to form, under the action of the heat of combustion, the aforesaid refractory mass. The invention also relates to a mixture of particles intended for a process of forming a coherent refractory mass on a surface by projecting the mixture and oxygen against the surface, the mixture comprising refractory particles and particles of a combustible substance which are capable of reacting exothermically with the oxygen in order to release sufficient heat to form, under the heat of combustion, the aforesaid refractory mass.
2. Background of the Art
If it is desired to form a refractory mass in situ on a surface, one can choose between two known principal processes.
According to the first of these processes, which is generally referred to as "ceramic welding" and which is illustrated in British Patents GB 1,330,894 (Glaverbel) and GB 2,170,191 (Glaverbel), which correspond respectively to U.S. Pat. Nos. 3,800,983 and 4,792,468, the disclosures of which are incorporated herein by reference, a coherent refractory mass is formed on a surface by projecting onto the latter a mixture of refractory particles and combustible particles in the presence of oxygen. The combustible particles are particles whose composition and granulometry are such that they react in an exothermic manner with the oxygen while forming a refractory oxide and while releasing the necessary heat for melting, at least superficially, the projected refractory particles. Aluminum and silicon are examples of such combustible substances. It is known that silicon, strictly speaking, is classified as a semiconductor, but because silicon behaves like certain metals (it is capable of undergoing considerable exothermic oxidation to form a refractory oxide), these combustible elements are referred to herein as combustible "metallic" substances for reason of simplicity. In general, it is recommended to perform the projection of particles in the presence of a high concentration of oxygen, for example, by using oxygen of commercial quality, as a gas carrier. In this manner, a coherent refractory mass is formed that adheres to the surface onto which the particles are projected. Because of the very high temperatures that the ceramic welding reaction can reach, it can penetrate slag which might be present on the surface of the refractory substance being treated, and it can soften or melt the surface of the refractory substance being treated in such a way that a good bond is produced between the treated surface and the newly formed refractory mass.
These known ceramic welding processes can be employed for forming a refractory article, for example, a block having a particular shape, but they are most widely used for forming coatings or for repairing bricks or walls and are particularly useful for repairing or reinforcing existing refractory structures, for example, for repairing walls or coating refractory equipment, such as furnace walls in glassmaking or coke furnaces.
This operation is generally performed when the refractory base is hot. This makes it possible to repair eroded refractory surfaces while the equipment remains substantially at its working temperature and, in certain cases, even while it is operating.
The second known process for forming a refractory mass on a surface is generally referred to as "flame spraying". It involves directing a flame to the site where a refractory mass is to be formed and spraying refractory powder across this flame. The flame is fed by a gaseous fuel or liquid or even coke powder. It is apparent that the efficient utilization of this flame spraying technique requires complete combustion of the fuel in order to generate the hottest flame possible and to attain maximum efficiency. In general, the temperature of the flame obtained with a flame spraying process is not so high as that which may be obtained with a ceramic welding technique, with the result that coherence of the formed refractory mass is not so good, and since the bond between the new refractory mass and the surface of the refractory base is formed at a lower temperature, this bond will not be as firm. Moreover, such a flame is less apt than a ceramic welding reaction to penetrate slag which might be present on the refractory surface being treated.
The composition of the mixture used in a ceramic welding process is generally chosen in such a way as to produce a repair mass which has a chemical composition similar or close to that of the basic refractory. This helps to ensure compatibility with and adhesion to the new material and the base material on which it is formed.
We have observed, however, that problems occur if it is desired to repair certain types of refractory structures even if a refractory mass of a chemical composition is formed which is similar to that of the basic refractory mass.
For example, repairing refractory surface structures having a silicon carbide base with a mixture containing primarily carbon and silicon particles and also particles of metallic combustible substances, produces a refractory mass which does not always demonstrate sufficient adhesion to the base refractory.
Refractories having a base of silicon carbide are used in certain metallurgy equipment, in particular, in blast furnaces in the iron industry or in zinc distillation columns. During the operation of this equipment, certain portions of the refractory structures may have a rather low minimum operating temperature, for example, on the order of 700.degree. C., and may additionally be subjected to significant variations in ambient temperature. It has been observed that the refractory masses produced by known techniques on these parts of refractory structures do not always demonstrate sufficient adhesion to the base refractory mass and, in certain cases, particularly when the repair is made on a block or a refractory wall whose temperature is low, the new refractory mass becomes completely separated from the base refractory mass and detaches itself during operation of the equipment.
Similar problems present themselves if one desires to repair refractory structures having a high density silica base (so named in order to distinguish them from traditional silica refractories whose density is lower) used in certain coke furnaces. Even though one may form a refractory similar in chemical composition to the base refractory mass, the new mass does not always adhere sufficiently and may even separate rapidly from the base refractory mass when the furnace is in operation.
It is an object of the present invention to solve the foregoing problems.